1. Introduction to Operating System (OS)
Operating Systems
Introduction to Operating System (OS)

2. Components of a Computer System

3. Components of a Computer System Contd…
Hardware – provides basic computing resources like CPU, memory, I/O devices.
Operating system – OS controls and coordinates the use of the hardware among the various application programs for different users.
Applications programs – They define the ways in which the system resources are used to solve the computing problems of the users.
Users (people, machines, other computers).

4. What is OS?
Operating System is a software, which makes a computer to actually work.
It is the software that enables all the programs we use.
The OS organizes and controls the hardware.
OS acts as an interface between the application programs and the machine hardware.
Examples: Windows, Linux, Unix and Mac OS, etc.,

5. USER View of OS
Ease to use
Efficient resource utilization

6. SYSTEM view of OS OS is a resource allocator
Manages all resources (CPU time, memory space, storage space, I/O and so on)
Decides between conflicting requests for efficient and fair resource use
OS is a control program
Controls execution of programs to prevent errors and improper use of the computer

7. Evolution of OS
Batch Processing
Multiprogramming
Timesharing

8. Batch Processing Early computers were extremely expensive Monitor
Important to maximize processor utilization
Monitor
Software that controls the sequence of events
Batch jobs together
Program returns control to monitor when finished

9. Batch Processing (Contd…)
The interface to the monitor was accomplished through Job Control Language (JCL).
For example, a JCL request could be to run the compiler for a particular programming language, then to link and load the program, then to run the user program.

10. Batch Processing (Contd…)
Hardware features: 
Memory protection: protect the memory area containing the monitor from the Utilities
Timer: prevents a job from monopolizing the system
Problems:
Bad utilization of CPU time - the processor stays idle while I/O devices are in use.

11. Multiprogramming
Multiprogramming is a technique to execute number of programs simultaneously by a single processor.
Number of processes reside in main memory at a time.
The OS picks and begins to execute one of the jobs in the main memory.
If any I/O wait happened in a process, then CPU switches from that job to another job.
Hence CPU in not idle at any time.

12. Multiprogramming (Contd…)
Processor must wait for I/O instruction to complete before preceding

13. Multiprogramming (Contd…)
When one job needs to wait for I/O, the processor can switch to the other job

14. Multiprogramming (Contd…)

15. Multiprogramming (Contd…)OS
Job 1
Job 2
Job 3
Job 4
Job 5
Advantages
Efficient memory utilization
Throughput increases
CPU is never idle, so performance increases.

16. Time Sharing Systems
Multiprogramming systems : several programs use the computer system
Time-sharing systems : several (human) users use the computer system interactively
Characteristics:
Using multiprogramming to handle multiple interactive jobs
Processor’s time is shared among multiple users
Multiple users simultaneously access the system through terminals

17. Time Sharing Systems (Contd…)
Time sharing, or multitasking, is a logical extension of multiprogramming
Multiple jobs are executed by switching the CPU between them
In this, the CPU time is shared by different processes, so it is called as “Time sharing Systems”.
Time slice is defined by the OS, for sharing CPU time between processes.
Examples: Multics, Unix, etc.,

18. Time Sharing Systems (Contd…)
Multiprogramming
Time Sharing
Principal objective
Maximize processor use
Minimize response time
Source of directives to operating system
Job control language commands provided with the job
Commands entered at the terminal

19. Types of OS Operating System can also be classified as
Single User Systems
Multi User Systems

20. Single User Systems Provides a platform for only one user at a time.
They are popularly associated with Desk Top operating system which run on standalone systems where no user accounts are required.
Example: DOS

21. MS-DOS execution
(a) At system startup (b) running a program

22. Multi-User Systems
Provides regulated access for a number of users by maintaining a database of known users.
Refers to computer systems that support two or more simultaneous users.
Another term for multi-user is time sharing.
Ex: All mainframes and  are multi-user systems.
Example: Unix

23. Operating System Services
The following are the services provided by the operating system.
User Interface
Program Execution
I/O Operations
File-system manipulation
Communications

24. Operating System Services (Contd…)
Error Detection
Resource Allocation
Accounting
Protection

25. User Interface
User interface - Almost all operating systems have a user interface (UI)
A user interface controls how to enter data and instructions and how information is displayed on the screen
With a graphical user interface (GUI), user interacts with menus and visual images
With a command-line interface, a user uses the keyboard to enter data and instructions

26. Program Execution
Operating system must be able to load a program into memory and to run that program.
The program must be able to end its execution, either normally or abnormally.

27. I/O Operations
A running program may require I/O, which may involve a file or an I/O device

28. File-System Manipulation
Programs need to read and write files and directories
The operating system handles the process of creating, deleting and searching files
Also list file Information, permission management

29. Communication
Every process needs to exchange information with another process
Communications may be implemented via shared memory or by the technique of message passing
Communications occurs in two ways
Within processes executing on the same computer
Between processes executing on different computers, tied together by a network

30. Error Detection Errors can happen at any time in the system
Errors can occur in the CPU, memory, I/O Devices, and in the user program
For each type of error, OS should take the appropriate action to ensure correct and consistent computing
Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system

31. Resource Allocation
Multiple jobs run at the same time so resources must be allocated to each one of them
Many different types of resources are managed by the operating system
Some (such as CPU cycles, main memory, and file storage) have special allocation code, whereas others (such as I/O devices) have general request and release code
The operating systems have various routines for different jobs in the system, for example CPU scheduling

32. Accounting
Need to keep track of number and type of resources allocated to processes/users
This tracking of user information can be used to find out the usage statistics.
These usage statistics are useful for the researchers who wish to re-configure the system to improve computing services.

33. Protection
Protection and Security is an important issue in multi-user computer systems.
Protection involves in ensuring that all access to system resources is controlled.
Security of the system requires user authentication, extends to defend external I/O devices from invalid access attempts
Access to the resources must be authenticated with a password.